Browsing by Author "Liu, Xiaolong"

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Boron-implanted black silicon photodiode with close-to-ideal responsivity from 200 to 1000 nm
(2023-06-21) Setälä, Olli; Chen, Kexun; Pasanen, Toni; Liu, Xiaolong; Radfar, Behrad; Vähänissi, Ville; Savin, Hele
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Detection of UV light has traditionally been a major challenge for Si photodiodes due to reflectance losses and junction recombination. Here we overcome these problems by combining a nanostructured surface with an optimized implanted junction and compare the obtained performance to state-of-the-art commercial counterparts. We achieve a significant improvement in responsivity, reaching near ideal values at wavelengths all the way from 200 to 1000 nm. Dark current, detectivity, and rise time are in turn shown to be on a similar level. The presented detector design allows a highly sensitive operation over a wide wavelength range without making major compromises regarding the simplicity of the fabrication or other figures of merit relevant to photodiodes.
Bridging the gap between surface physics and photonics
(2024-04) Laukkanen, Pekka; Punkkinen, Marko; Kuzmin, Mikhail; Kokko, Kalevi; Liu, Xiaolong; Radfar, Behrad; Vähänissi, Ville; Savin, Hele; Tukiainen, Antti; Hakkarainen, Teemu; Viheriälä, Jukka; Guina, Mircea
A2 Katsausartikkeli tieteellisessä aikakauslehdessä
Use and performance criteria of photonic devices increase in various application areas such as information and communication, lighting, and photovoltaics. In many current and future photonic devices, surfaces of a semiconductor crystal are a weak part causing significant photo-electric losses and malfunctions in applications. These surface challenges, many of which arise from material defects at semiconductor surfaces, include signal attenuation in waveguides, light absorption in light emitting diodes, non-radiative recombination of carriers in solar cells, leakage (dark) current of photodiodes, and light reflection at solar cell interfaces for instance. To reduce harmful surface effects, the optical and electrical passivation of devices has been developed for several decades, especially with the methods of semiconductor technology. Because atomic scale control and knowledge of surface-related phenomena have become relevant to increase the performance of different devices, it might be useful to enhance the bridging of surface physics to photonics. Toward that target, we review some evolving research subjects with open questions and possible solutions, which hopefully provide example connecting points between photonic device passivation and surface physics. One question is related to the properties of the wet chemically cleaned semiconductor surfaces which are typically utilized in device manufacturing processes, but which appear to be different from crystalline surfaces studied in ultrahigh vacuum by physicists. In devices, a defective semiconductor surface often lies at an embedded interface formed by a thin metal or insulator film grown on the semiconductor crystal, which makes the measurements of its atomic and electronic structures difficult. To understand these interface properties, it is essential to combine quantum mechanical simulation methods. This review also covers metal-semiconductor interfaces which are included in most photonic devices to transmit electric carriers to the semiconductor structure. Low-resistive and passivated contacts with an ultrathin tunneling barrier are an emergent solution to control electrical losses in photonic devices.
Carrier Recombination in Femtosecond Laser-Textured Silicon
(2022-04-19) Forssell, Ida
Sähkötekniikan korkeakoulu | Bachelor's thesis
Contactless analysis of surface passivation and charge transfer at the TiO 2-Si interface
(2024-05-03) Khan, Ramsha; Liu, Xiaolong; Vähänissi, Ville; Ali-Löytty, Harri; Pasanen, Hannu P.; Savin, Hele; Tkachenko, Nikolai V
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Transition metal oxides are pivotal in enhancing surface passivation and facilitating charge transfer (CT) in silicon based photonic devices, improving their efficacy and affordability through interfacial engineering. This study investigates TiO 2/Si heterojunctions prepared by atomic layer deposition (ALD) with different pre-ALD chemical and post-ALD thermal treatments, exploring their influence on the surface passivation and the correlation with the CT at the TiO 2-Si interface. Surface passivation quality is evaluated by the photoconductance decay method to study the effective carrier lifetime, while CT from Si to TiO 2 is examined by transient reflectance spectroscopy. Surprisingly, the as-deposited TiO 2 on HF-treated n-Si (without interfacial SiO x) demonstrates superior surface passivation with an effective lifetime of 1.23 ms, twice that of TiO 2/SiO x/n-Si, and a short characteristic CT time of 200 ps, tenfold faster than that of TiO 2/SiO x/n-Si. Post-ALD annealing at temperatures approaching the TiO 2 crystallization onset re-introduces the SiO x layers in HF-treated samples and induces chemical and structural changes in all the samples which decrease passivation and prolong the CT time and are hence detrimental to the photonic device performance.
Excellent Responsivity and Low Dark Current Obtained with Metal-Assisted Chemical Etched Si Photodiode
(2023-04-01) Chen, Kexun; Setälä, Olli; Liu, Xiaolong; Radfar, Behrad; Pasanen, Toni; Serue, Michael; Heinonen, Juha; Savin, Hele; Vähänissi, Ville
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Metal-assisted chemical etched (MACE; also known as MacEtch or MCCE) nanostructures are utilized widely in the solar cell industry due to their excellent optical properties combined with a simple and cost-efficient fabrication process. The photodetection community, on the other hand, has not shown much interest toward MACE due to its drawbacks, including insufficient surface passivation, increased junction recombination, and possible metal contamination, which are especially detrimental to p-n photodiodes. Here, we aim to change this by demonstrating how to fabricate high-performance MACE p-n photodiodes with above 90% external quantum efficiency (EQE) without external bias voltage at 200-1000 nm and dark current less than 3 nA/cm2 at -5 V using industrially applicable methods. The key is to utilize an induced junction created by an atomic layer deposited (ALD) highly charged Al2O3 thin film that simultaneously provides efficient field-effect passivation and full conformality over the MACE nanostructures. Achieving close to ideal performance demonstrates the vast potential of MACE nanostructures in the fabrication of high-performance low-cost p-n photodiodes.
Hyperdoping-regulated room-temperature NO2 gas sensing performances of black silicon based on lateral photovoltaic effect
(2023-05-01) Wang, Wenjing; Li, Hua; Liu, Xiaolong; Ma, Shengxiang; Zhao, Yang; Dong, Binbin; Li, Yuan; Ning, Xijing; Zhao, Li; Zhuang, Jun
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Black silicon co-hyperdoped with sulfur and nitrogen in different ratios is prepared by femtosecond laser-assisted chemical etching in the mixed atmosphere of SF6 and NF3 with varying gas pressure ratios. Their room-temperature NO2 gas sensing capability is studied systematically, in which the photocurrent as a readout signal is generated by the lateral photovoltaic effect of black silicon under an asymmetrical light illumination. These co-hyperdoped black silicon exhibits high response, fast response/recovery, ultrawide detection range from 29 ppb to 2000 ppm, excellent selectivity and acceptable long-term durability over 3 months. Moreover, NO2 gas sensing performances are effectively tuned or optimized by deliberately changing the co-doping ratio of sulfur and nitrogen, as different photovoltaic characteristics are induced by changes in morphology and structural defects resulting from different hyperdoping. Specifically, ultra-high relative gas response (~3955%@20 ppm NO2) and superior selectivity are obtained at the SF6/NF3 pressure ratio of 56/14, while faster response/recovery time (17 s/ 47 s@20 ppm NO2) and response photocurrent with a weaker disturbance by humidity are given by the samples with SF6/NF3 of 7/63 and 63/7, respectively. Therefore, such black silicon material has good potential to meet different application needs.
Impact of post-ion implantation annealing on Se-hyperdoped Ge
(2024-07-22) Liu, Xiaolong; Mc Kearney, Patrick; Schäfer, Sören; Radfar, Behrad; Berencen, Yonder; Kentsch, Ulrich; Vähänissi, Ville; Zhou, Shengqiang; Kontermann, Stefan; Savin, Hele
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Hyperdoped germanium (Ge) has demonstrated increased sub-bandgap absorption, offering potential applications in the short-wavelength-infrared spectrum (1.0-3.0 μm). This study employs ion implantation to introduce a high concentration of selenium (Se) into Ge and investigates the effects of post-implantation annealing techniques on the recovery of implantation damage and alterations in optical properties. We identify optimal conditions for two distinct annealing techniques: rapid thermal annealing (RTA) at a temperature of 650 °C and ultrafast laser heating (ULH) at a fluence of 6 mJ/cm2. The optimized ULH process outperforms the RTA method in preserving high doping profiles and achieving a fourfold increase in sub-bandgap absorption. However, RTA leads to regrowth of single crystalline Ge, while ULH most likely leads to polycrystalline Ge. The study offers valuable insights into the hyperdoping processes in Ge for the development of advanced optoelectronic devices. © 2024 Author(s).
Impact of Pulse Duration on the Properties of Laser Hyperdoped Black Silicon
(2024-02-27) Mc Kearney, Patrick; Schäfer, Sören; Liu, Xiaolong; Paulus, Simon; Lebershausen, Ingo; Radfar, Behrad; Vähänissi, Ville; Savin, Hele; Kontermann, Stefan
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The impact of three different pulse durations (100 fs, 1, and 10 ps) on the formation of laser hyperdoped black silicon with respect to surface morphology, sub-bandgap absorptance, the sulfur concentration profile, and the effective minority carrier lifetime after Al2O3 surface passivation is investigated. The current flow behavior is compared through the hyperdoped layer by I–V measurements after hyperdoping with different pulse durations. For conditions that give the same absolute sub-bandgap absorptance, an increase in pulse duration from 100 fs to 10 ps results in a shallower sulfur concentration profile. Findings are explained by an increasing ablation threshold from 0.19 J cm−2 for a pulse duration of 100 fs to 0.21 J cm−2 for 1 ps and 0.34 J cm−2 for 10 ps. The formation of an equally absorbing layer with a shallower doping profile results in a reduction in contact and/or sheet resistance. Despite the higher local sulfur concentration, the samples show no decrease in carrier lifetime measured by quasi-steady-state photoconductance decay on Al2O3 surface-passivated samples. The investigation shows that longer pulses of up to 10 ps during laser hyperdoping of silicon result in advanced layer properties that promise to be beneficial in a potential device application.
Infrared Absorption in Laser Processed Black Silicon
(2021-09-05) Pälikkö, Elmeri
Sähkötekniikan korkeakoulu | Bachelor's thesis
Measuring Wide Spectral Absorption of Laser Textured Germanium
(2024-05-07) Halmela, Julius
Sähkötekniikan korkeakoulu | Bachelor's thesis
Germanium shows high potential as a new material in optoelectronic applications, which are applications that detect and absorb light. Its affordability and nontoxicity enhance its appeal compared to currently used materials. The materials are textured using a femtosecond laser. By varying the laser power and scan speed the aim is to discover the optimal laser parameters to maximize the absorptance. During laser irradiation, dopants can be introduced on the surface of the material and their effect is studied with different coating thicknesses. Additionally, some samples undergo thermal annealing to test their thermal stability. The irradiation process induces structural changes on the surface of the germanium, resulting in a higher absorption. This new surface texture scatters light in multiple directions, requiring a spectrophotometer with an integrating sphere setup to capture and measure all the scattered light. The device effectively minimizes measurement errors and ensures normalized results. The experimental results revealed that laser processing can increase both aboveand below-bandgap absorption. Furthermore, the coating has a minimal effect on the absorptance if the laser parameters are more intense. Moreover, the absorptance is significantly increased for higher laser power and lower laser scan speed. Such laser treatment shows a remarkable increase in absorptance for longer wavelengths, notably below the bandgap of germanium. While thermal annealing causes a small decrease in absorptance, it has other benefits such as structural stability, useful in optoelectronic applications.
Mid-long wavelength infrared absorptance of hyperdoped silicon via femtosecond laser microstructuring
(2022-01-17) Sun, Haibin; Liu, Xiaolong; Zhao, Li; Jia, Jianxin; Jiang, Changhui; Xiao, Jiamin; Chen, Yuwei; Xu, Long; Duan, Zhiyong; Rao, Peng; Sun, Shengli
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Hyperdoped silicon (hSi) fabricated via femtosecond laser irradiation has emerged as a promising photoelectric material with strong broadband infrared (IR) absorption. In this work, we measured the optical absorptance of the hSi in the wavelength of 0.3–16.7 µm. Unlike the near to mid wavelength IR absorption, the mid-long wavelength IR (M–LWIR) absorption is heavily dependent on the surface morphology and the dopants. Furthermore, calculations based on coherent potential approximation (CPA) reveal the origin of free carrier absorption, which plays an important role in the M–LWIR absorption. As a result, a more comprehensive picture of the IR absorption mechanism is drawn for the optoelectronic applications of the hSi.
Millisecond-Level Minority Carrier Lifetime in Femtosecond Laser-Textured Black Silicon
(2022-08-15) Liu, Xiaolong; Radfar, Behrad; Chen, Kexun; Pälikkö, Elmeri; Pasanen, Toni; Vähänissi, Ville; Savin, Hele
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Femtosecond laser-textured black silicon (fs-bSi) is known to suffer from heavy minority carrier recombination resulted from laser irradiation. In this paper, we demonstrate that the thermal annealing step, generally used to recover the crystal damage, could improve the minority carrier lifetime of the fs-bSi wafers only from 8 μs to 12 μs, even when using as high temperature as 800 °C. However, with an optimized wet chemical etching process, we obtain a high minority carrier lifetime of 2 ms without sacrificing the optical properties of the samples, i.e., the absorptance remains above 90% in the studied wavelength range (250–1100 nm). Increasing the etching time further leads to a total recovery of the lifetime up to 10.5 ms, which proves that the damage originating from the fs-laser texturing extends only to the near-surface layer (a few μm) of silicon.
Optoelectronic properties of black silicon fabricated by femtosecond laser in ambient air: exploring a large parameter space
(2023-03-01) Radfar, Behrad; Chen, Kexun; Setälä, Olli; Vähänissi, Ville; Savin, Hele; Liu, Xiaolong
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We study the surface morphology, optical absorption (400–1100 nm), and carrier lifetime of black silicon fabricated by femtosecond (fs) laser in air. We explore a large laser parameter space, for which we adopt a single parameter ξ to describe the cumulative fluence delivered to the sample. We also study the laser-oxidized surface layer by measuring its photoluminescence spectra and comparing its effect on the aforementioned properties. Our study in a broad range of ξ is instructive in choosing laser parameters when targeting different applications.
(oral talk) Hyperdoping germanium with titanium via femtosecond laser processing
(2024) Liu, Xiaolong; Gnatyuk, Dmytro; Vähänissi, Ville; Savin, Hele
Abstract
(oral talk) Surface Passivation and Charge Transfer at TiO2/Si Interface
(2023) Liu, Xiaolong; Khan, Ramsha; Pasanen, Hannu; Ali-Löytty, Harri; Vähänissi, Ville; Valden, Mika; Tkachenko, Nikolai V.; Savin, Hele
Abstract
Titanium dioxide (TiO2) is widely employed in photoelectric applications. This study investigates the surface passivation and charge transfer dynamics of a TiO2 layer grown on Si by atomic layer deposition (ALD). We analyze the passivation effect of TiO2 by measuring carrier lifetime and surface barrier height, and study charge transfer dynamics at TiO2/Si using contactless transient reflectance spectroscopy. Results show that a proper chemical treatment on p-Si prior to ALD enhances both the charge transfer properties and passivation effects.
Perspectives on Black Silicon in Semiconductor Manufacturing: Experimental Comparison of Plasma Etching, MACE and Fs-Laser Etching
(2022-08) Liu, Xiaolong; Radfar, Behrad; Chen, Kexun; Setälä, Olli; Pasanen, Toni; Yli-Koski, Marko; Savin, Hele; Vähänissi, Ville
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
In semiconductor manufacturing, black silicon (bSi) has traditionally been considered as a sign of unsuccessful etching. However, after more careful consideration, many of its properties have turned out to be so superior that its integration into devices has become increasingly attractive. In devices where bSi covers the whole wafer surface, such as solar cells, the integration is already rather mature and different bSi fabrication technologies have been studied extensively. Regarding the integration into devices where bSi should cover only small selected areas, existing research focuses on device properties with one specific bSi fabrication method. Here, we fabricate bSi patterns with varying dimensions ranging from millimeters to micrometers using three common bSi fabrication techniques, i.e., plasma etching, metal-assisted chemical etching (MACE) and femtosecond-laser etching, and study the corresponding fabrication characteristics and resulting material properties. Our results show that plasma etching is the most suitable method in the case of µm-scale devices, while MACE reached surprisingly almost the same performance. Femtosecond-laser has potential due to its maskless nature and capability for hyperdoping, however, in this study its moderate accuracy, large silicon consumption and spreading of the etching damage outside the bSi region left room for improvement.
Properties of Black Silicon Layers Fabricated by Different Techniques for Solar Cell Applications
(2023-11-15) Ayvazyan, Gagik; Ayvazyan, Karen; Hakhoyan, Levon; Liu, Xiaolong
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Black silicon (BS) layers coated with passivation films are widely used as antireflective frontal surfaces for solar cells. The most common BS fabrication techniques are reactive ion etching (RIE), metal-assisted chemical etching, and laser-induced processing. Herein, the structural and optical properties, as well as the minority carrier lifetime, of BS are compared with and without atomic layer deposited HfO 2 passivation films produced by the above formation methods. The antireflection behavior of the samples is discussed based on the light trapping effect and the change in the BS refractive index from air to the bulk of crystalline Si. Finally, test solar cells are manufactured, and their photovoltaic parameters are studied. The comparison results show that RIE is the most preferred in all technical respects. The features of using different BS fabrication techniques from the solar cell manufacturing point of view are analyzed.
Synergic Effect of N and Se Facilitates Photoelectric Performance in Co-Hyperdoped Silicon
(2024-10) Sun, Haibin; Liu, Xiaolong; Xu, Caixia; Xu, Long; Chen, Yuwei; Yang, Haima; Yang, Xing; Rao, Peng; Sun, Shengli; Zhao, Li
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Femtosecond-laser-fabricated black silicon has been widely used in the fields of solar cells, photodetectors, semiconductor devices, optical coatings, and quantum computing. However, the responsive spectral range limits its application in the near- to mid-infrared wavelengths. To further increase the optical responsivity in longer wavelengths, in this work, silicon (Si) was co-hyperdoped with nitrogen (N) and selenium (Se) through the deposition of Se films on Si followed by femtosecond (fs)-laser irradiation in an atmosphere of NF3. The optical and crystalline properties of the Si:N/Se were found to be influenced by the precursor Se film and laser fluence. The resulting photodetector, a product of this innovative approach, exhibited an impressive responsivity of 24.8 A/W at 840 nm and 19.8 A/W at 1060 nm, surpassing photodetectors made from Si:N, Si:S, and Si:S/Se (the latter two fabricated in SF6). These findings underscore the co-hyperdoping method’s potential in significantly improving optoelectronic device performance.
Tailoring Femtosecond-Laser Processed Black Silicon for Reduced Carrier Recombination Combined with >95% Above-Bandgap Absorption
(2022-01-17) Liu, Xiaolong; Radfar, Behrad; Chen, Kexun; Pasanen, Toni; Vähänissi, Ville; Savin, Hele
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The femtosecond-pulsed laser processed black silicon (fs-bSi) features high absorptance in a wide spectral range but suffers from high amount of laser induced damage as compared to bSi fabricated by other methods. Here, we aim to minimize the charge carrier recombination in the fs-bSi caused by laser damage as indicated by the sub-bandgap absorption and as quantified by the carrier lifetime, while maintaining high absorption in the above-bandgap. The effect of the laser parameters, including the focal position, the average power, and the scan speed are systematically studied by characterizing the surface morphology, the absorptance spectra, and the minority-carrier recombination lifetime. For the surface passivation of fs-bSi we use the well-established atomic layer deposited (ALD) Al2O3. The results show that with the tailored laser parameters, high average absorptance of about 96% in the visible range and minority carrier lifetime of 54 μs at the injection level of Δn = 1 ∙ 1015 cm−3 can be obtained simultaneously. This work paves the way towards high-performance broadband optoelectronic devices based on surface passivated fs-bSi.
Wetting Properties of Black Silicon Layers Fabricated by Different Techniques
(2024-08) Ayvazyan, Gagik; Hakhoyan, Levon; Vardanyan, Arman; Savin, Hele; Liu, Xiaolong
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The wettability of black silicon (BSi) layers fabricated by reactive ion etching (RIE), metal-assisted chemical etching (MACE), and laser-induced etching (LIE) techniques is studied. The contact angles of wetting on the samples with deionized water and methylammonium iodide-based perovskite solutions are determined. It is found that the element composition and the enlargement area factor of BSi layers have a significant effect on their wettability. When tested with water, the RIE and MACE BSi layers exhibit hydrophobic properties, while the LIE BSi layer demonstrates hydrophilic properties due to the SiOx-rich surface structures. It is also shown that aging leads to a decrease in the water contact angle. Upon exposure to perovskite solution droplets, BSi layers become highly lyophilic. Based on the Wenzel and Cassie–Baxter models, the mechanisms responsible for the wetting states of the fabricated samples are identified. In the results obtained, valuable insights are provided into the potential of using these layers in tandem perovskite/silicon solar cells.